Re: What would you do? (Older male teacher with young female student)
As a father of small girls (about 4 yrs old), Ido realize your reasons for concern, but I am not sure the of the nature of the situation. It seems to me, neither are you.
There are two possibilities here:
* The innocent one: He & She were attracted to eachother despite the age difference, both waited until she came of age. Nothing could have affected the relationship in the dojo or outside of it, and it has nothing to do with Aikido. Their relationship was "meant to be" regrdless of your prejustice about it, and would likly continue until he dies of old age.
And, most important, he is not looking for a younger replacement of her. Further, if the relationship breaks he may find anyone else (young or old).
* The predatory one: He is a predator, who found some young victim who enjoyed his attention, but he keeps looking and will not be satisified.
Another question to be asked here, is why did you not interfere while she was younger? If you thought you shouldn't, why do you think others should have (including the dojo cho)? Are you sure the dojo-cho did not look into this and got convinced it is the first type?
Aikido community is not safer inherently than any other, you can still have adults of mixed sex and age mingling and befriending each other. Few might be predators, others may be just looking for a soul mate without limits. It is the responsibility of all members of the community to stop the predators, regardless of rank.